The present invention relates generally to fuel cell systems and more specifically to fuel cell systems containing recuperators or heat exchangers with spiral flow and methods of operating same.
FIGS. 1-9 illustrate a prior art fuel cell system described in U.S. Published Application 2007/0196704 A1 published on Aug. 23, 2007 (filed as Ser. No. 11/656,563 on Jan. 23, 2007) titled Integrated Solid Oxide Fuel Cell And Fuel Processor and incorporated herein by reference in its entirety. Specifically, with reference to FIGS. 1, 2A, 2B and 3A, an integrated fuel cell unit 10 is shown in form of an integrated solid oxide fuel cell (“SOFC”)/fuel processor 10 having a generally cylindrical construction. The unit 10 includes an annular array 12 of eight (8) fuel cell stacks 14 surrounding a central axis 16, with each of the fuel cell stacks 14 having a stacking direction extended parallel to the central axis 16, with each of the stacks having a face 17 that faces radially outward and a face 18 that faces radially inward. As best seen in FIG. 3A the fuel cell stacks 14 are spaced angularly from each other and arranged to form a ring-shaped structure about the axis 16. Because there are eight of the fuel cell stacks 14, the annular array 12 could also be characterized as forming an octagon-shaped structure about the axis 16. While eight of the fuel cell stacks 14 have been shown, it should be understood that the invention contemplates an annular array 12 that may include more than or less than eight fuel cell stacks.
With reference to FIG. 1, the unit 10 further includes an annular cathode recuperator 20 located radially outboard from the array 12 of fuel stacks 14, an annular anode recuperator 22 located radially inboard from the annular array 12, a reformer 24 also located radially inboard of the annular array 12, and an annular anode exhaust cooler/cathode preheater 26, all integrated within a single housing structure 28. The housing structure 28 includes an anode feed port 30, an anode exhaust port 32, a cathode feed port 34, a cathode exhaust port 36, and an anode combustion gas inlet port 37. An anode exhaust combustor (typically in the form an anode tail gas oxidizer (ATO) combustor), shown schematically at 38, is a component separate from the integrated unit 10 and receives an anode exhaust flow 39 from the port 32 to produce an anode combustion gas flow 40 that is delivered to the anode combustion gas inlet 37. During startup, the combustor 38 also receives a fuel flow (typically natural gas), shown schematically by arrow 41. Additionally, some of the anode exhaust flow may be recycled to the anode feed port 30, as shown by arrows 42. In this regard, a suitable valve 43 may be provided to selectively control the routing of the anode exhaust flow to either the combustor 38 or the anode feed port 30. Furthermore, although not shown, a blower may be required in order to provide adequate pressurization of the recycled anode exhaust flow 42. While FIGS. 1, 2A and 2B are section views, it will be seen in the later figures that the components and features of the integrated unit 10 are symmetrical about the axis 16, with the exception of the ports 34, 36 and 37.
With reference to FIG. 1 and FIG. 2A, the cathode flows will be explained in greater detail. As seen in FIG. 1, a cathode feed (typically air), shown schematically by arrows 44, enters the unit 10 via the port 34 and passes through an annular passage 46 before entering a radial passage 48. It should be noted that as used herein, the term “radial passage” is intended to refer to a passage wherein a flow is directed either radially inward or radially outward in a generally symmetric 360 degree pattern. The cathode feed 44 flows radially outward through the passage 48 to an annular passage 50 that surrounds the array 12 and passes through the cathode recuperator 20. The cathode feed 44 flows downward through the annular passage 50 and then flows radially inward to an annular feed manifold volume 52 that surrounds the annular array 12 to distribute the cathode feed 44 into each of the fuel cell stacks 14 where the cathode feed provides oxygen ions for the reaction in the fuel cell stacks 14 and exits the fuel cell stacks 14 as a cathode exhaust 56. The cathode exhaust 56 then flows across the reformer 24 into an annular exhaust manifold area 58 where it mixes with the combustion gas flow 40 which is directed into the manifold 58 via an annular passage 60. In this regard, it should be noted that the combustion gas flow 40 helps to make up for the loss of mass in the cathode exhaust flow 56 resulting from the transport of oxygen in the fuel cell stacks 14. This additional mass flow provided by the combustion gas flow 40 helps in minimizing the size of the cathode recuperator 20. The combined combustion gas flow 40 and cathode exhaust 56, shown schematically by arrows 62, exits the manifold 58 via a central opening 64 to a radial passage 66. The combined exhaust 62 flows radially outward through the passage 66 to an annular exhaust flow passage 68 that passes through the cathode recuperator 20 in heat exchange relation with the passage 50 to transfer heat from the combined exhaust 62 to the cathode feed 44. The combined exhaust 62 flows upward through the annular passage 68 to a radial passage 70 which directs the combined exhaust 62 radially inward to a final annular passage 72 before exiting the unit 10 via the exhaust port 36.
With reference to FIG. 1 and FIG. 2B, an anode feed, shown schematically by arrows 80, enters the unit 10 via the anode feed inlet port 30 preferably in the form of a mixture of recycled anode exhaust 42 and methane. The anode feed 80 is directed to an annular passage 82 that passes through the anode recuperator 22. The anode feed 80 then flows to a radial flow passage 84 where anode feed 80 flows radially outward to an annular manifold or plenum 86 that directs the anode feed into the reformer 24. After being reformed in the reformer 24, the anode feed 80 exits the bottom of reformer 24 as a reformats and is directed into an integrated pressure plate/anode feed manifold 90. The feed manifold 90 directs the anode feed 80 to a plurality of stack feed ports 92, with one of the ports 92 being associated with each of the fuel cell stacks 14. Each of the ports 92 directs the anode feed 80 into a corresponding anode feed/return assembly 94 that directs the anode feed 82 into the corresponding fuel cell stack 14 and collects an anode exhaust, shown schematically by arrows 96, from the corresponding stack 14 after the anode feed reacts in the stack 14. Each of the anode feed/return assemblies 94 directs the anode exhaust 96 back into a corresponding one of a plurality of stack ports 98 in the pressure plate/manifold 90 (again, one port 98 for each of the fuel cell stacks 14). The manifold 90 directs the anode exhaust 96 radially inward to eight anode exhaust ports 100 (again, one for each stack 14) that are formed in the pressure plate/manifold 90. The anode exhaust 96 flows through the ports 100 into a plurality of corresponding anode exhaust tubes 102 which direct the anode exhaust 96 to a radial anode exhaust flow passage 104. The anode exhaust 96 flows radially inward through the passage 104 to an annular flow passage 106 that passes downward through the anode recuperator 22 in heat exchange relation with the flow passage 82. The anode exhaust 96 is then directed from the annular passage 106 upward into a tubular passage 108 by a baffle/cover 110 which is preferably dome-shaped. The anode exhaust 96 flows upwards through the passage 108 before being directed into another annular passage 112 by a baffle/cover 114, which again is preferably dome-shaped. The annular passage 112 passes through the anode cooler 26 in heat exchange relation with the annular cathode feed passage 46. After transferring heat to the cathode feed 44, the anode exhaust 96 exits the annular passage 112 and is directed by a baffle 116, which is preferably cone-shaped, into the anode exhaust port 32.
With reference to FIGS. 3A, 3B, the reformer 24 is provided in the form of an annular array 280 of eight tube sets 282, with each tube set 282 corresponding to one of the fuel cell stacks 14 and including a row of flattened tubes 284. In this regard, it should be noted that the number of tubes 284 in the tube sets 282 will be highly dependent upon the particular parameters of each application and can vary from unit 10 to unit 10 depending upon those particular parameters.
FIG. 3C is intended as a generic figure to illustrate certain construction details common to the cathode recuperator 20, the anode recuperator 22, and the anode cooler 26. The construction of each of these three heat exchangers basically consists of three concentric cylindrical walls A,B,C that define two separate flow passages D and E, with corrugated or serpentine fin structures G and H provided in the flow passages D and E, respectively, to provide surface area augmentation of the respective flow passages. Because the heat transfer occurs through the cylindrical wall B, it is preferred that the fins G and H be bonded to the wall B in order to provide good thermal conductivity, such as by brazing. On the other hand, for purposes of assembly and/or allowing differential thermal expansion, it is preferred that the fins G and H not be bonded to the cylindrical walls A and C. For each of the heat exchangers 20, 22 and 26, it should be understood that the longitudinal length and the specific geometry of the fins G and H in each of the flow paths D and E can be adjusted as required for each particular application in order to achieve the desired output temperatures and allowable pressure drops from the heat exchangers.
Turning now to FIG. 4A-D, the anode cooler 26 includes a corrugated or serpentine fin structure 300 to provide surface area augmentation for the anode exhaust 96 in the passage 112, a corrugated or serpentine fin structure 302 that provides surface area augmentation for the cathode feed flow 44 in the passage 46, and a cylindrical wall or tube 304 to which the fins 300 and 302 are bonded, preferably by brazing, and which serves to separate the flow passage 46 from the flow passage 112. As best seen in FIG. 4B, a cylindrical flow baffle 306 is provided on the interior side of the corrugated fin 300 and includes the dome-shaped baffle 114 on its end in order to define the inner part of flow passage 112. A donut-shaped flow baffle 308 is also provided to direct the cathode feed 44 radially outward after it exists the flow passage 46. The cone-shaped baffle 116 together with the port 32 are attached to the top of the tube 304, and include a bolt flange 310 that is structurally fixed, by a suitable bonding method such as brazing or welding, to the port 32, which also includes a bellows 311 to allow for thermal expansion between the housing 28 and the components connected through the flange 310. As seen in FIG. 4C, the above-described components can be assembled as yet another subassembly that is bonded together, such as by brazing.
In reference to FIGS. 1 and 4D, it can be seen that the anode recuperator 22 includes a corrugated or serpentine fin structure 312 (best seen in FIG. 8) in the annular flow passage 82 for surface area augmentation for anode feed 80. As best seen in FIG. 1, the anode recuperator 22 further includes another corrugated or serpentine fin structure 314 in the annular flow passage 106 for surface augmentation of the anode exhaust 96.
As best seen in FIG. 4D, corrugated fins 312 and 314 are preferably bonded to a cylindrical wall of tube 316 that serves to separate the flow passages 82 and 106 from each other, with the dome-shaped baffle 110 being connected to the bottom end of the wall 316. Another cylindrical wall or tube 320 is provided radially inboard from the corrugated fin 314 (not shown in FIG. 4D, but in a location equivalent to fin 300 in cylinder 304 as seen in FIG. 4B) to define the inner side of the annular passage 106, as best seen in FIG. 4D. As seen in FIG. 2A, an insulation sleeve 322 is provided within the cylindrical wall 320 and a cylindrical exhaust tube 324 is provided within the insulation sleeve 322 to define the passage 108 for the anode exhaust 96. Preferably, the exhaust tube 324 is joined to a conical-shaped flange 328 provided at a lower end of the cylindrical wall 320. With reference to FIG. 4D, another cylindrical wall or tube 330 surrounds the corrugated fin 312 to define the radial outer limit of the flow passage 82 and is connected to the inlet port 30 by a conical-shaped baffle 332. A manifold disk 334 is provided at the upper end of the wall 316 and includes a central opening 336 for receiving the cylindrical wall 320, and eight anode exhaust tube receiving holes 338 for sealingly receiving the ends of the anode exhaust tubes 102, with the plate 308 serving to close the upper extent of the manifold plate 334 in the assembled state.
With reference to FIGS. 2B and 4E, a heat shield assembly 350 is shown and includes an inner cylindrical shell 352 (shown in FIG. 2B), an outer cylindrical shell 354, an insulation sleeve 356 (shown in FIG. 2B) positioned between the inner and outer shells 352 and 354, and a disk-shaped cover 358 closing an open end of the outer shell 350. The cover 358 includes eight electrode clearance openings 360 for through passage of the electrode sleeves 211. As seen in FIG. 4E, the heat shield assembly 350 is assembled over an insulation disk 361 the outer perimeter of the assembled array 12 of fuel cells 14 and defines the outer extent of the cathode feed manifold 52. The heat shield 350 serves to retain the heat associated with the components that it surrounds. FIG. 5 shows the heat shield assembly 350 mounted over the stacks 14.
With reference to FIG. 1 and FIG. 6, the cathode recuperator 20 includes a corrugated or serpentine fin structure 362 to provide surface enhancement in the annular flow passage 68 for the combined exhaust 62, a corrugated or serpentine fin structure 364 to provide surface enhancement in the annular flow passage 50 for the cathode feed 44, and a cylindrical tube or wall 366 that separates the flow passages 50 and 68 and to which the fins 362 and 364 are bonded. A disk-shaped cover plate 368 is provided to close the upper opening of the cylindrical wall 366 and includes a central opening 370, and a plurality of electrode clearance openings 372 for the passage of the electrode sleeve 211 therethrough. A cylindrical tube or sleeve 376 is attached to the cover 368 to act as an outer sleeve for the anode cooler 26, and an upper annular bolt flange 378 is attached to the top of the sleeve 376. A lower ring-shaped bolt flange 380 and an insulation sleeve 382 are fitted to the exterior of the sleeve 376, and a cylindrical wall or shield 384 surrounds the insulation sleeve 382 and defines an inner wall for the passage 72, as best seen in FIGS. 1 and 6.
With reference to FIG. 7, the components of FIG. 6 are then assembled over the components shown in FIG. 5 with the flange 378 being bolted to the flange 310.
With reference to FIG. 4A, the outer housing 28 is assembled over the remainder of the unit 10 and bolted thereto at flange 380 and a flange 400 of the housing 28, and at flange 402 of the assembly 237 and a flange 404 of the housing 28, preferably with a suitable gasket between the flange connections to seal the connections.
FIG. 9 is a schematic representation of the previously described integrated unit 10 showing the various flows through the integrated unit 10 in relation to each of the major components of the integrated unit 10. FIG. 9 also shows an optional air cooled anode condenser 460 that is preferably used to cool the anode exhaust flow 39 and condense water therefrom prior to the flow 39 entering the combustor 38. If desired, the condenser may be omitted. FIG. 9 also shows a blower 462 for providing an air flow to the combustor 38, a blower 464 for providing the cathode feed 44, and a blower 466 for pressurizing the anode recycle flow 42. If desired, in an alternate embodiment of the unit 10 shown in FIG. 9 also differs from the previously described embodiment shown in FIG. 1 in that an optional steam generator (water/combined exhaust heat exchanger) 440 is added in order to utilize waste heat from the combined exhaust 62 to produce steam during startup. In this regard, a water flow 442 is provided to a water inlet port 444 of the heat exchanger 440, and a steam outlet port directs a steam flow 448 to be mixed with the anode feed 80 for delivery to the anode feed inlet port 30.